Conventional methods of recycling waste rubber from tires generally involve size reduction and mechanical removal of the steel belts, followed by defibering with chemicals such as zinc or calcium chlorides, and exposure to high temperature for prolonged periods of time to achieve some carbon-carbon bond cleavage to form three-dimensional, crosslinked rubber fragments which can be recompounded to form lower-grade products such as automobile floor mats. Reclaimed rubber fragments can also be used in relatively small amounts to form products such as semi-pneumatic tires and inner tubes. It is also known to use conventional reclaimed rubber as an additive to asphalt to inhibit flow at elevated temperatures while also improving low-temperature flexibility. However, the relatively meager demand for reclaimed rubber from scrap tires for use in asphalt, floor mats and other low-grade products is overwhelmingly outweighed by the ever burgeoning accumulation of scrap tires at landfill and other waste gathering sites.
Accumulation of scrap tires represents a major environmental problem by presenting a serious risk of potential environmental damage by ignition thereof, for example by lightening or by vandals. Another disadvantage is that tires do not decompose under conditions encountered at waste gathering sites and, therefore, represent an ever increasing volume of material occupying dwindling amounts of available space for waste disposal. At the same time, scrap tires and other scrap rubber can represent a significant source of energy and valuable chemical feedstocks.
Ground rubber materials have been used directly as a fuel and/or in combination with other fuels. However, the direct combustion of waste rubber is generally unacceptable due primarily to the difficulty of obtaining complete combustion. In particular, the amount of mechanical energy needed to achieve a sufficient particle size reduction to effect complete combustion of the rubber would make it uneconomical and impractical to use waste rubber as a fuel. As a result, the use of waste rubber as a fuel has generally resulted in incomplete combustion, undesirable stack emissions, fouling of the combustion apparatus, and high amounts of residue and ash.
U.S. Pat No. 4,740,270 to Roy discloses a method for the treatment of used tires by vacuum pyrolysis to produce liquid and gaseous hydrocarbons and a solid carbonaceous material. The pyrolysis is carried out at a temperature of about 350.degree. to about 415.degree. C. and under a subatmospheric pressure of less than about 35 mm Hg to produce a pyrolytic oil typically containing about 60weight percent hydrocarbon oils, about 38 weight-percent solid carbonaceous material, and about 2 weight percent gaseous hydrocarbons. The pyrolytic oil is reported to have a heating value of about 10,200 kcal per kg. The process has the disadvantage that only about 60 percent of the rubber waste is converted to a liquid fuel.